The control center and the terminal devices communicate with each other preferably by radio. The exchange of information among the components of the telecommunications system occurs by way of a protocol that is structured in the time domain in frames. In this protocol, both the exchange of signaling information and the exchange of useful information occurs with the aid of a sequence of frames, preferably lasting the same length of time. The terminal device must be associated before the terminal device may communicate with the control center. Various parameters are negotiated during such an association. The exchange of information between a terminal and the control center is organized, for example, using a channel access protocol. The channel access protocol is also called Media Access Control (MAC). With regard to the association of a terminal with the control center, it is important that each terminal have a specific MAC address attached to it, by which the terminal is uniquely identified within the domain of the control center.
Communications among the components of a telecommunications system, i.e. both between the control center and a terminal device and between two terminal devices, is carried out by frames. This means communication occurs in accordance with the present invention with the assistance of MAC frames, for example, of a fixed duration. Each of these MAC frames provides transmission capacity in the direction from the terminal devices to the control center (uplink) and in the direction from the control center to the terminal devices (downlink). Moreover, a MAC frame may also provide transmission capacity between various terminal devices (point to point). A process for implementing such frame-based transmission is described in German Published Patent Application No. 197 26 120.
Power-saving devices are used to make a longer operating time possible, particularly in the case of portable terminal devices, for example in Global System for Mobile Communications (GSM) equipment. These devices are easily incorporated into GSM, since GSM is a type of circuit-switched communication. In other words, it has only two states: “connected” and “disconnected.” In the “connected” state data are exchanged continuously between the network and the subscriber. In the “disconnected” state the terminal device must regularly determine which base station the terminal device is receiving best and whether an incoming call is waiting for it. Incoming calls are transmitted from all base stations in the roaming area and the terminal device responds to the base station it receives best at the time, so it may take the call. The terminal device itself does not signal until it leaves the roaming area. In this case, it must register in the new roaming area.
The most important features of the power-saving devices as regards the present invention are the following:                The terminal device signals to the control center its desire to enter the sleep mode; a sleep interval parameter describes this desire in greater detail.        The control center responds with a message confirming, if necessary, a different sleep interval parameter.        In accordance with the stipulated sleep interval parameter, the terminal device does not listen for a certain number of MAC frames, after which a MAC frame is sent that is once again heard by the terminal device.        The control center knows when the terminal device is listening and may transmit a message to the terminal device during this particular MAC frame, telling it to monitor subsequent messages. If this does not occur, then the terminal device goes into the sleep mode once again.        This is repeated until either the terminal, i.e. the terminal device, or the control center has a message to transmit.        
Between the MAC frames stipulated as the frames during which the terminal will listen, the control center is unable to reach the terminal. Thus, if the control center has a message for a sleeping terminal, it must wait until the terminal is ready once again to receive messages. However, the control center knows precisely when the terminal may be reached once again.